A spark plug is used in an internal combustion engine or the like and typically includes an insulator having an axial bore extending in the axial direction. A center electrode is provided at the front end side of the axial bore, and a terminal electrode is provided at the rear end side of the axial bore. The terminal electrode includes a head portion which is exposed at the rear end of the insulator and to which a plug cap or the like is to be attached for supply of power. The terminal electrode further includes a rodlike leg portion which is inserted into the axial bore of the insulator and whose front end subportion is fixed to the insulator by means of a glass seal or the like. A cylindrical metallic shell is fixed to an outer circumference of the insulator. A ground electrode is joined to a front end portion of the metallic shell. A predetermined voltage is applied to the spark plug via the plug cap or the like attached to the terminal electrode, thereby generating spark discharges between the center electrode and the ground electrode.
In recent years, in view of environmental protection and the like, the fuel consumption of an internal combustion engine has been severely regulated. In order to prevent a drop in output while at the same time meeting the regulation of fuel consumption, a reduction in the displacement of the internal combustion engine, a higher degree of compression and/or a higher degree of supercharging has been employed.
An internal combustion engine which employs a higher degree of compression and/or a higher degree of supercharging requires a higher voltage for spark discharge. However, increasing an applied voltage may cause current to leak between the terminal electrode and the metallic shell in such a manner as to creep (i.e., migrate) onto the surface of the insulator, potentially resulting in the occurrence of misfire associated with discharge abnormality. In order to prevent the leakage of current (so-called flashover), increasing the length (to, for example, about 35 mm) of a portion (a rear trunk portion) of the insulator extending between the terminal electrode and the metallic shell was conceived (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-155839). In this case, in association with an increase in the length of the rear trunk portion of the insulator, the leg portion of the terminal electrode to be inserted into the insulator increases in length.